Migratory birds, whose deaths are being linked to avian flu, have likely played a big role in spreading the disease, reports Keith Bradsher in Thailand.

Tucked among broad-leafed banana groves and emerald-green rice paddies in west-central Thailand, the bird sanctuary here is a jungle oasis where migratory ducks, cranes and other wildfowl spend their winters in safety. They come from as far as India and Siberia.

But many of the birds have been dying of what Thai scientists suspect is bird flu. World Health Organisation officials say migratory birds have probably played a central role in spreading the disease. Their infected droppings dry up, turn to dust and are inhaled by other birds.

While chickens and other domesticated fowl succumb easily to avian influenza, migratory birds are hardier and can be infected for long periods and travel great distances while showing few ill effects.

The wildfowl deaths here appear to be another ominous sign of the virulence of the epidemic that scientists now fear.

Pakistan announced on Monday that millions of chickens had been infected with what officials described as a mild strain of the influenza virus. Additional reports of large-scale chicken deaths from a mysterious illness have come from Burma, Laos, Bangladesh and Saudi Arabia.

South Korea, Japan, Vietnam, Cambodia, Thailand and Indonesia have confirmed cases of the deadly Type A influenza virus strain H5N1, while Taiwan and Pakistan have reported different, milder strains that appear to pose less threat to people.

Thailand announced on Monday that it had a second confirmed human case of avian influenza, and that it suspected 10 more people, five of them dead, had avian influenza. Cambodia said two children were suspected of having the disease, Vietnam is investigating more than a dozen possible human cases, and there have been reports of a mysterious viral outbreak in Bangladesh.

The medically documented cases so far appear to have resulted from birds infecting people, but the fear is that the disease might evolve into a form that could pass easily from person to person, said Dr Bjorn Melgaard, the WHO's chief representative in Thailand.

Workers in most countries with confirmed outbreaks are culling millions of chickens in the hope of stopping the spread. But with so many countries reporting cases, and with migratory birds and possibly chicken traders carrying the disease ever further, United Nation officials warn that it will not be easy to curb.

Environmentalists in Thailand warned against trying to cull wild birds or eliminate their habitats, saying it would not be possible to cull enough birds to stamp out the disease, and that a cull could endanger rare species.

The outbreak scares health officials because if a person with human influenza becomes infected with the avian version, the viruses may swap genetic material. That could produce a virus that is easily transmitted from person to person, although nobody knows if it will.

Previous flu pandemics, like the Spanish influenza epidemic of 1918 and 1919 and the Asian and Hong Kong flu epidemics of the 1950s and 1960s, are believed to have started in birds.